The present invention relates to a ZrO.sub.2 sintered body containing Y.sub.2 O.sub.3 and Nd.sub.2 O.sub.3 as stabilizers of the crystal structure of ZrO.sub.2 (hereinafter referred to simply as "stabilizers"), and more particularly to a ZrO.sub.2 sintered body having high toughness for use in tools such as dies and cutters, structural parts, decorating parts, etc.
Recently, partially stabilized zirconia (simply "PSZ") has been getting much attention as a high-strength, high-toughness ceramic. This PSZ undergoes stress-induced transformation, that is a phenomenon in which tetragonal ZrO.sub.2 metastable at room temperature is transformed into stable monoclinic ZrO.sub.2 under the influence of an outside force. Specifically, an outside force is absorbed as energy for the transformation, and ZrO.sub.2 undergoes a volume increase, preventing the generation of cracks which cause fracture. This provides ZrO.sub.2 with high strength and toughness.
The PSZ which is presently being used presently is Y.sub.2 O.sub.3 -PSZ, which contains a small amount of Y.sub.2 O.sub.3 as a stabilizer. This Y.sub.2 O.sub.3 -PSZ is fully described in "Microstructurally Developed Toughening Mechanisms in Ceramics," Technical Report No. 3, F. F. Lange, Rockwell International, July 1978. In addition, Japanese Patent Publication No. 61-21184 (Japanese Patent Laid-Open No. 56-134564) discloses that Y.sub.2 O.sub.3 -PSZ has high strength and high durability at 200.degree.-300.degree. C. when satisfying the conditions that the Y.sub.2 O.sub.3 content is 2-7 mol %, that its crystal structure is mainly composed of a tetragonal phase, and that its average crystal grain size is 2 .mu.m or less.
Further, "Microstructurally Developed Toughening Mechanisms in Ceramics, Technical Report No. 7, F. F. Lange, Rockwell International, October 1979 discloses a ZrO.sub.2 --Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 sintered body, and Japanese Patent Publication No. 61-59265 (Japanese Patent Laid-Open No. 58-32066) discloses a zirconia sintered body containing Y.sub.2 O.sub.3, etc. as stabilizers and consisting essentially of 40-99.5 wt % of tetragonal ZrO.sub.2 and 0.5-60 wt % of Al.sub.2 O.sub.3.
These zirconia sintered bodies have improved hardness and strength because the solution and dispersion of Al.sub.2 O.sub.3 in ZrO.sub.2 lowers the temperature at which tetragonal ZrO.sub.2 is transformed into the monoclinic phase, preventing the grain growth of ZrO.sub.2, thereby increasing the content of tetragonal ZrO.sub.2 and also enhancing the sliding resistance of ZrO.sub.2 grains along the grain boundaries.
On the other hand, various attempts have been made to provide methods of producing such zirconia sintered bodies. For instance, Japanese Laid-Open No. 60-54972 discloses a method of producing ZrO.sub.2 ceramics having improved strength by pressing ZrO.sub.2 powder containing a certain amount of stabilizers such as Y.sub.2 O.sub.3, etc. and sintering it to a relative density of 93% or more, and then subjecting the sintered body to hot isostatic pressing (simply HIP).
Japanese Patent Publication No. 61-59267 (Japanese Patent Laid-Open No. 58-36976) gives consideration the starting material powder, proposing the use of starting material powder produced by a coprecipitation method in which each component of ZrO.sub.2, stabilizers and Al.sub.2 O.sub.3 can ideally be dispersed, and sintering it to provide a sintered body having a uniform structure with substantially no micropores.
Further, Japanese Patent Laid-Open No. 60-86073 discloses the use of the above HIP method using starting material powder produced by a coprecipitation method.
Various attempts have been made to use stabilizers other than Y.sub.2 O.sub.3. For instance, Technical Report of Tohoku University Metal Materials Laboratory, No. 12, pp. 19-21 (March, 1987) shows the effects of stabilizing tetragonal ZrO.sub.2 by a system (3.5 mol %, Ln.sub.2 O.sub.3 Sc.sub.2 O.sub.3, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Ce.sub.2 O.sub.3, Pr.sub.2 O.sub.3, Nd.sub.2 O.sub.3.) It is indicated that among the above oxides, Nd.sub.2 O.sub.3 shows the largest next to Y.sub.2 O.sub.3.
However, with respect to the use of the conventional PSZ in tools such as dies and cutters, it is inferior to competing cemented carbide in toughness and hardness. Accordingly, PSZ has been used in extremely limited applications, and it has been considered difficult to find sufficient applications.
Particularly, when Al.sub.2 O.sub.3 is added, the toughness of ZrO.sub.2 ceramics markedly decreases, although its strength and hardness increase. Accordingly, there is a need for the development of ZrO.sub.2 ceramics having excellent strength, toughness and hardness.